Goto the acp panel
www.yoursite.com/adm/
Click styles>templates>
click edit next to prosilver (if that is your current template)
choose overall_footer.html and click select template file
post your adsense code at the bottom or make a div and add it in between the div you make
update IGNORE all the text below (not worth the effort in phpbb3)
This is a modification (Mod) for phpBB3 forums, to use this modification you need a working installation of phpBB3. For instructions how to install this modification see here. For getting support see the links at the right of the page.
http://code.google.com/p/phpbb3-ad-management/
http://www.phpbb.com/mods/installing/
Extract the contents of the zip to your desktop. One of the files will be called install.xml, this has all the installation instructions in it.
http://www.phpbb.com/customise/db/mod/advertisement_management/support/installing-t_85856